1. Field of the Invention
The present invention relates to apparatus for heating a layer of dielectric material with waves of a radio frequency, preferable of the high frequency range, the apparatus containing a radio frequency generator, electrodes of a load and feed lines between the radio frequency generator and the load, the working frequency of the generator being balanced to a series resonance of the capacitor formed by the electrode, the feed lines and the generator and in which a parallel resonance circuit with the resonant frequency of the series resonant circuit is effective, simultaneously, for the radio frequency generator.
2. Description of the Prior Art
Apparatus of the type set forth above are known, in general, and are used for drying leather, for example. It has turned out in large area devices which have very low electrical resistance, however, that extraordinarily high currents are necessary in order to achieve the desired effect. For facilitation, therefore, one or more inductances are connected in parallel to the load, these inductances forming a high-resistant parallel resonant circuit together with the capacitive load impedance. Furthermore, the inductance of the feed is augmented by a series capacitance to form a series oscillating circuit with the frequency of the generator, so the reactive impedance of the feed is compensated for the generator frequency. The circuit formed by the load and by the inductances connected in parallel, however, change with respect to its resonant frequency during the heating-up operation when the apparatus is operated. The generator frequency which had been finally adjusted, according to the prior art, therefore, enables the full exploitation of the generator power only for a portion of the processing time in the cases under consideration, in that the feed satisfies the effective power set in the load and in the inductances. The resonant circuit, however, becomes detuned to such a degree during the heat-up operation, particularly when drying leather or similar articles, so that a considerable reactive power must be exerted in addition to the effective power.